This invention relates to a tungsten carbide (hereinafter indicated by WC)-base hard alloy having toughness and abrasion resistance possessed by WC-base hard alloys as well as excellent high-temperature strength, hot-impact resistance and hot-fatigue resistance, which is particularly suitable for use as a material for hot working apparatus members for which these characteristics are required, such as hot-rolling rolls, hot-rolling guide rollers and hot-forging dies, etc.
As materials for hot working apparatus members as mentioned above, tool steels or cast steels conventionally used are frequently replaced in recent years by WC-base hard alloy, comprising WC having a high value of high-temperature hardness as disperse phase bound with binding metals composed principally of Co. As such WC-base hard alloys, there have been known those of the WC-Co system, the WC-Co-Ni system, and the WC-Co-Ni-Cr system. However, while a WC-base hard alloy has excellent toughness and abrasion resistance on the one hand, it does not have sufficient high-temperature strength. Therefore, as in the case of hot-rolling rolls for steel-wire rods, when the roll surfaces are subjected to heating at a high temperature under application of pressure by running steel wire rods at 1,000.degree. to 1,100.degree. C., and the roll surfaces are also chilled with water, the roll surfaces will suffer from thermal cracks or coarsening under such conditions of repeated cycles of heating and cooling. WC-Co-Ni system and WC-Co-Ni-Cr-system hard alloys, while having better characteristics than a WC-Co system hard alloy, have a drawback in that they are readily chipped, which is believed to be due particularly to thermal cracks under severe conditions of low speed and high load, thus failing to exhibit satisfactory performance.
Meanwhile, there has also been proposed a WC-Co-Ni-Al system hard alloy, comprising a disperse phase of WC, and 20 to 70% (by weight, hereinafter the same unless otherwise noted) of Co, 0.1 to 10% Ni, and 0.05 to 5% of Al as binder metals, and further containing, if desired, Cr.sub.3 C.sub.2, TaC and TiC (Japanese Laid-open Patent Application No. 90511/75). This hard alloy is also still not satisfactory in mechanical characteristics such as transverse rupture strength, tensile strength, hardness, etc., especially at high temperatures. Further, because of its high content of Co, the alloy has poor oxidation resistance and corrosion resistance. Thus, this alloy is also not satisfactory as a hard alloy for hot-working apparatus members.